Ah! the joys of gardening. Just know that your plants are living and something can go wrong. It’s life, it happens. As a gardener, you first need to know how not to panic. This is where your resources come in. Hopefully, you have started collecting your books, articles and newsletters, you have joined some forums online. You know where to go to get answers and help. Know that you will find an answer.
Below you will find an interesting article by Kristi Waterworth over on the Home Guides website, on what could be causing the brown spots on your watermelons, and what you can do about it.
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Blossom End Rot
Blossom end rot is a common problem of watermelons, caused by a severe lack of available calcium during fruiting. Because calcium is relatively immobile once taken up by a plant, it’s difficult for a watermelon vine to redistribute this nutrient to newly developing fruits. Even if calcium was readily available at the beginning of the season, water stress can force the majority of calcium circulating in plant tissues into the leaves due to rapid transpiration.[/su_quote]
Go over to the Home Guides website to read the entire article.